1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling the speed ratio of a continuously variable transmission for a motor vehicle.
2. Discussion of the Prior Art
An example of a hydraulic control apparatus for controlling the speed ratio of a vehicle continuously variable transmission is disclosed in laid-open Publication No. 61-48658 of unexamined Japanese Patent Application. Here, the speed ratio "r" is defined as a ratio Nin/Nout, where Nin represents the speed of the input shaft of the transmission, and Nout represents the speed Nout of the output shaft. The hydraulic control apparatus disclosed in this publication uses a shift control valve which includes: a valve spool adapted to receive an input shaft speed pressure corresponding to the speed of an input shaft of the continuously variable transmission so that the spool is biased by this pressure in the shift-up direction; an actuator plunger adapted to receive via a spring a thrust corresponding to the amount of operation of an accelerator pedal of the vehicle so that the spool is biased by this thrust in the shift-down direction; an adjusting spring; and a first and a second plunger which are received in the spool and the actuator plunger, respectively, and which are adapted to compress the adjusting spring interposed therebetween when a line pressure of the hydraulic circuit of the apparatus is lowered. When the line pressure is lowered as the speed ratio is reduced from the maximum value, the speed ratio is controlled so that the input shaft speed pressure increases with an increase in the compressive force of the adjusting spring, so as to maintain equilibrium of forces acting on the valve spool in the opposite directions. Consequently, the rate of increase in the input shaft speed of the transmission increases with a rise in the running speed of the vehicle. This arrangement adapted to increase the input shaft speed at a rate sufficiently higher than the rate of an initial rise in the vehicle speed not only provides a high degree of initial acceleration of the vehicle, but also assures reduced operating noise of the engine with the engine speed limited to a comparatively low level when the speed ratio of the transmission is near the maximum value.
In the thus constructed known hydraulic control apparatus for a vehicle continuously variable transmission, the input shaft speed of the transmission is changed based on the line pressure which varies as a function of the speed ratio, as the speed ratio is changed from the maximum value to the minimum value. In this arrangement, the input shaft speed is changed relative to the vehicle running speed, irrespective of a change in the amount of opening of the throttle valve (amount of operation of the accelerator pedal), over the entire range of the speed ratio of the transmission between the maximum and minimum values. Therefore, there is a tendency that the input shaft speed considerably increases when the vehicle speed has been raised to a comparatively high level. Thus, the known hydraulic control apparatus does not permit sufficiently high fuel economy of the vehicle.